Leukocyte, fibroblast and lymphoblastoid interferons were removed from crude tissue culture fluids by columns of anti-leukocyte interferon attached to Sepharose-4B. As specific activity of interferon in biological fluid is very low, attempts were also made to prepare radioactive interferons in tissue culture medium. Following purification by affinity chromatography, gel filtration, ion exchange chromatography, isoelectric focusing and slab gel electrophoresis, the isolated minute amounts of radioactive interferon were subjected to sequence analysis. The results obtained with tritiated interferon were inconclusive because of insufficient radioactivity. Currently, we are engaged in the large scale production of lymphoblastoid interferon in conjunction with the Frederick Cancer Research Center, Frederick, Md. Following the induction by interferon with NDV-B1 virus lymphastoid cells are removed from the medium by filtration using an array of filters and Hyflo-Super-Cel. The virus present in the filtrate is killed by boiling for 10 minutes at pH 2. The conditions for the concentration of interferon by SDSppt are in progress in this laboratory.